The present invention relates generally to the conversion of a broad spectrum of materials and especially to a method for the hydrothermal treatment of organics containing or generating inorganic compounds such as salts or oxides, at supercritical temperature and pressure conditions, or at supercritical temperatures and elevated, yet subcritical pressures.
The process of wet oxidation has been used for the treatment of aqueous streams for over thirty (30) years. In general, the process involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous stream at elevated temperatures and pressures. The resultant "combustion" of organic or inorganic oxidizable materials occurs directly within the aqueous phase.
A wet oxidation process is typically characterized by operating pressures of 30 bar to 250 bar (440 psia-3,630 psia) and operating temperatures of one hundred fifty degrees Celsius to three hundred seventy degrees Celsius (150.degree. C.-370.degree. C.), for which liquid and gas phases coexist for aqueous media. Since gas phase oxidation is quite slow at these temperatures, the reaction is primarily carried out in the liquid phase. The reactor operating pressure is typically maintained at or above the saturated water vapor pressure, so that at least part of the water is present in liquid form. Even in the liquid phase, however, reaction times for substantial oxidation are on the order of one (1) hour. In many applications, reaction times of this length are unacceptable.
In addition to unacceptably long reaction times, the utility of conventional wet oxidation is limited by several factors. These include: the degree of oxidation attainable; an inability to adequately oxidize refractory compounds; and the lack of usefulness for power recovery due to the low temperature of the process. For these reasons, there has been considerable interest in extending wet oxidation to higher temperatures and pressures. For example, U.S. Pat. No. 2,944,396, which issued Jul. 12, 1960 to Barton et al., discloses a process wherein an additional second oxidation stage is accomplished after wet oxidation. In the Barton process, unoxidized volatile combustibles which accumulate in the vapor phase of the first stage wet oxidation reactor are sent to complete their oxidation in the second stage. This second stage is operated at temperatures above the critical temperature of water, about three hundred seventy four degrees Celsius (374.degree. C.).
A significant development in the field occurred with the issuance of U.S. Pat. No. 4,338,199, to Modell on Jul. 6, 1982. The Modell '199 patent discloses a wet oxidation process which has come to be known as supercritical water oxidation ("SCWO"). As the name SCWO implies, in some implementations of the SCWO process, oxidation occurs essentially entirely at conditions which are supercritical in both temperature (&gt;374.degree. C.) and pressure (&gt;about 3,200 psi or 220 bar). Importantly, SCWO has been shown to give rapid and complete oxidation of virtually any organic compound in a matter of seconds at five hundred degrees Celsius to six hundred fifty degrees Celsius (500.degree. C.-650.degree. C.) and 250 bar. During this oxidation, carbon and hydrogen in the oxidized material form the conventional combustion products carbon dioxide ("CO.sub.2 ") and water. When chlorinated hydrocarbons are involved, they give rise to hydrochloric acid ("HCl"), which will react with available cations to form chloride salts. Due to the adverse effects of HCl, alkali may be intentionally added to the reactor to avoid high, corrosive concentrations of hydrochloric acid in the reactor and especially in the cooldown equipment following the reactor. When sulfur oxidation is involved, the final product in SCWO is a sulfate anion. This is in contrast to normal combustion, which forms gaseous sulfur dioxide ("SO.sub.2 "). As in the case of chloride, alkali may be intentionally added to avoid high concentrations of sulfuric acid. Similarly, the product of phosphorus oxidation is phosphate anion.
At typical SCWO reactor conditions densities are in the range of 0.1 g/cc, so water molecules are considerably farther apart than they are in ambient liquid water. Hydrogen bonding, a short-range phenomenon, is almost entirely disrupted, and the water molecules lose the ordering responsible for many of liquid water's characteristic properties. In particular, solubility behavior is closer to that of high pressure steam than to liquid water. Smaller polar and nonpolar organic compounds, with relatively high volatility, will exist as vapors at typical SCWO conditions, and hence will be completely miscible with supercritical water. Gases such as N.sub.2, O.sub.2, and CO.sub.2 show similar complete miscibility. Larger organic compounds and polymers will hydrolyze to smaller molecules at typical SCWO conditions, thus resulting in solubilization via chemical reaction. The loss of bulk polarity by the water phase has striking effects on normally water-soluble salts, as well. In particular, because they are no longer readily solvated by water molecules, salts frequently precipitate out as solids which can deposit on process surfaces and cause fouling of heat transfer surfaces or blockage of the process flow.
A process related to SCWO known as supercritical temperature water oxidation ("STWO") can provide similar oxidation effectiveness for certain feedstocks but at lower pressure. This process has been described in U.S. Pat. No. 5,106,513, issued Apr. 21, 1992 to Hong, and utilizes temperatures in the range of six hundred degrees Celsius (600.degree. C.) and pressures between 25 bar to 220 bar. On the other hand, for the treatment of some feedstocks, the combination of temperatures in the range of four hundred degrees Celsius to five hundred degrees Celsius (400.degree. C.-500.degree. C.) and pressures of up to 1,000 bar (15,000 psi) have proven useful to keep certain inorganic materials from precipitating out of solution (Buelow, S. J., "Reduction of Nitrate Salts Under Hydrothermal Conditions," Proceedings of the 12.sup.th International Conference on the Properties of Water and Steam, ASME, Orlando, Fla., September, 1994). The various processes for oxidation in an aqueous matrix are referred to collectively as hydrothermal oxidation, if carried out at temperatures between about three hundred seventy-four degrees Celsius to eight hundred degrees Celsius (374.degree. C.-800.degree. C.), and pressures between about 25 bar to 1,000 bar. Similar considerations of reaction rate, solids handling, and materials corrosion apply also to the related process of hydrothermal reforming, in which an oxidant is largely or entirely excluded from the system in order to form products which are not fully oxidized. The processes of hydrothermal oxidation and hydrothermal reforming will hereinafter be jointly referred to as "hydrothermal treatment."
One of the key issues which must be addressed in the application of hydrothermal oxidation is the means by which incoming feed material is brought up to reaction temperature. A typical approach is the use of a heater or heat exchanger, in which the feed material passes through an elongated tube or tubes to absorb heat. For many feed materials, however, the possibility of organic char formation, inorganic scaling, and corrosivity can make the operation of such a heat exchanger or heater very difficult. Use of such a heating scheme has the further effect of increasing the specific energy of the feedstock, so that feeds must be processed at more dilute levels. This is appropriate for feeds dilute in organic, but a disadvantage for concentrated organic feeds. For example, a cold feed with a heating value of 1,800 Btu/lb will reach an adiabatic oxidation temperature of about six hundred degrees Celsius (600.degree. C.) when air or oxygen is used as the oxidant. If, however, this same feed has been preheated to about three hundred seventy-five degrees Celsius (375.degree. C.), the approximate temperature at which rapid reaction commences, it is limited to a heating value of 900 Btu/lb to reach six hundred degrees Celsius (600.degree. C.). Thus, the organic content of the preheated feed can only be about half that of the un-preheated feed.
A second key issue pertaining to hydrothermal oxidation processes is the means by which feed streams containing or generating sticky solids are handled. It is well-known that such feed streams can result in solids accumulation within and eventual plugging of the process equipment. Sticky solids are generally comprised of salts, such as halides, sulfates, carbonates, and phosphates. One of the earliest designs for handling such solids on a continuous basis is given in U.S. Pat. No. 4,822,497. Reaction is carried out in a vertically oriented vessel reactor. Solids form as the reaction proceeds and are projected and fall into a cooler brine zone maintained at the bottom of the reactor. The sticky solids re-dissolve in the brine and may be continually drawn off from the reactor. Solids separation is achieved because only the fraction of the process stream necessary for solids dissolution and transport is withdrawn as brine. The balance of the process stream, which is frequently the largest portion, is caused to reverse flow to an upward direction within the reactor, and is withdrawn from the reactor top section. By this means, it becomes possible to recover a hot, nearly solids-free stream from the process. To minimize entrainment of solid particles in the upward flow within the reactor, the velocity is kept to a low value by using a large cross-section reactor vessel. Experience has shown that while a large fraction of the sticky solids is transferred into the brine zone, a certain portion also adheres to the vessel walls, eventually necessitating an online or off-line cleaning procedure. It is also likely that the amount of solids deposited on the reactor walls is exacerbated by the reversing flow pattern within the reactor. Thus, this design may have drawbacks for the processing of certain feedstocks.
In light of the above, it is an object of the present invention to provide a system and method for hydrothermal treatment which continuously and reliably handles waste streams containing or generating significant quantities of sticky solids while minimizing the need for flushing of the reactor walls. Another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment in a continuous online process wherein the reactor residence time of the material being reacted is less than one (1) minute. Still another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment which uses a simple geometry in order to minimize the surface area subject to solids deposition and adherence. Yet another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment which is easy to implement, simple to use, and cost effective.